1. Field of the Invention
The present invention relates to a method of increasing the useful length of heating elements at low temperatures, and more specifically the useful life of elements of the molybdenum-silicide and the molybdenum-tungsten silicide type, including different alloys of these basic materials. Elements of this nature are produced by applicants in a number of forms.
2. Description of the Related Art
When heating elements formed from molybdenum-silicide, molybdenum-tungsten-silicide, and alloys thereof are operated at relatively low temperatures, for example at temperatures around 400-550° C., no protective scale of silicon dioxide, so-called glass skin, will form, as is the case when operating the elements at high temperatures. Instead, the elements are subjected to so-called pest attack, meaning that a non-protective layer of a mixture of MoO3 and SiO2 forms on the surfaces of the elements. This mixture is porous and readily disintegrates, resulting in a significant shortening of the useful life of the elements.
However, there are applications in which such elements are, nevertheless, the best alternative. One example in this regard is found in the heating of LPCVD (Low Pressure Chemical Vapor Deposition) chambers in the manufacture of electronic circuits.
One way of improving the low temperature properties of such heating elements is to pre-oxidize the elements at a temperature of about 1500° C. or higher, so as to form SiO2 scale. Such scale will slow down the formation of pest. Pest occurs as a result of MoSi2 and O2 forming MoO3 and SiO2. This oxide mixture is relatively porous and hence will afford no protection against continued oxidation.
Such methods, however, do not afford satisfactory protection against pest formation.
Another method is described in Swedish Patent Specification No. 0001846-5. This method involves causing the water content of the atmosphere surrounding the elements to be kept low when said elements are operated. This method provides satisfactory protection against pest formation.
The method according to the present invention significantly increases the length of life of such elements.